staging: comedi: addi_apci_3120: factor DMA setup out of apci3120_cyclic_ai()

For aesthetics, factor the DMA setup code out of apci3120_cyclic_ai().

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Reviewed-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>